In recent years, full HD (High Definition) video contents have become increasingly popular, and chances of viewing video images on a display having a full HD resolution have increased. Note that the full HD indicates a video image made up of 1920×1080 pixels.
When a low-resolution video image recorded on a DVD or the like is viewed on a full-HD display, an enlargement process needs to be performed to convert the video image to high resolution. However, the conversion of images from low resolution to high resolution has a problem that the images are blurred by the enlargement process.
As the process for sharpening images as a solution to the above problem, an edge enhancement filter process and a super resolution process are known. Note that the super resolution process mentioned here is a process for correcting details of images with such a precision that cannot be obtained by the edge enhancement filter process (for example, see Non-Patent Literatures 1, 2 and 3).
Meanwhile, some super resolution processes perform the correction process in units of pixels. In that case, when a process-target pixel includes noise, even the noise may be sharpened (enhanced).
In general, a video content is recorded onto a recording medium after it is encoded. A so-called encoding noise occurs during the encoding/decoding process. Such a noise often occurs in a high-frequency region (peripheral of an edge part) in images constituting a video image.
In view of the above, a method in which the edge enhancement filter process instead of the super resolution process is performed on the high-frequency region has been proposed (see, for example, Non-Patent Literature 4).
Another proposed method is a combined filter process for performing a combined process of a noise removal filter process and the edge enhancement filter process to restrict the influence of the above-described noise (see, for example, Patent Literature 1).
Note that there is also proposed a method for measuring the amount of noise and changing the noise removal level value of the noise removal filter to increase the noise removal efficiency (see, for example, Patent Literature 2).